The prior art for electromagnetic drillstring telemetry is based upon inductive (toroidal) or direct coupling of a source signal carrying the downhole sensor information to the drillstring and surrounding formation. Toroidal coupled systems induce a modulated electric current on the drillstring by means of electromagnetic coupling between a (primary) toroidal coil encircling a conductive mandrel connected to the drillstring, and a secondary coil comprising the drillstring, and surrounding formation. The modulated current, which is induced in the secondary, flows along the drillstring and drilling fluid, and through the formation in a pattern, which is governed by the electrical conductivity(s) of the drillstring and drilling fluid, and surrounding formation. The flow of current on the drillstring and through the formation is measured by a receiving apparatus at the surface.
The receiving apparatus is either inductively coupled to the modulated current through a transformer or directly coupled by sensing the potential difference (voltage) produced by the flow of modulated current between electrodes "grounded" at the surface. A previous patent (U.S. Pat. No. 4,181,014 to Zuvela et al.) describes several means of signal reception using sub-surface electrodes connected to the surface by insulated conductors. (See also U.S. Pat. No. 4,980,682 to Klein et al.)
The operation of the inductively coupled (toroidal) downhole transmitter-receiver (transceiver) is enhanced by insulating gaps in the downhole transceiver sub-assembly to isolate the toroidal primary coil from the surrounding drill collar (which would otherwise provide a direct short to the secondary, if it were not electrically isolated). The toroidal-inducing coil encircles an electrically conducting mandrel, which is mechanically and electrically connected to the upper and lower sections of drillstring. The toroidal sub-assembly and associated electronics are designed to provide impedance matching between the source circuitry and the load of the drillstring-formation circuit (U.S. Pat. No. 4,496,174 to McDonald et al., 1985).
In the prior art, the source impedance may be matched with the load using matching transformers (U.S. Pat. No. 2,389,241 to Silverman, 1944; U.S. Pat. No. 4,691,203 to Rubin, 1987). Matching transformers and associated complex electrical circuitry are employed to match the impedance of the downhole sub-assembly electronics to the very low impedance associated with the small gaps necessary to maintain the mechanical stability of the downhole transceiver sub-assembly. One of the herein inventors has previously patented an apparatus for electromechanical impedance matching (U.S. Pat. No. 5,130,706 to Van Steenwyk, 1992).
Transformer coupled electric-field telemetry systems require that the signal information be transmitted by various forms of modulation of a carrier signal. Pulse modulated systems have been described (U.S. Pat. No. 3,046,474 to Arps, 1962; U.S. Pat. No. 4,015,234 to Krebs, 1977); but these systems have required the generation of a very high-voltage pulse by means of capacitor discharge to overcome the poor impedance match between the downhole transmitter and the drillstring-formation load impedance.
More recently, a low-voltage, low-impedance, current generator has been described (U.S. Pat. No. 5,270,703 to Guest). It should be noted that none of these methods for coupling a pulse to the drillstring-formation path are suited to a talk-down capability. See also U.S. Pat. No. 4,684,946 to Geoservice.